Twitter HACKED, please help me ?!?

Theres someone in my school who has hacked my twitter account, but i can still access it. They tweet while im online. I know its someone from my school because theyve been insulting my friends and they tagged my friends and was tweeting obvious and familiar tweets.. they are on my twitter account from their phone, is there anyway I can log out, Out of their phone ?!?!?! please help me.. !!! THANKS!